Cannondale combines CAD and composites

March 2, 1998 Design News

CAD AWARD WINNER EXCELLENCE IN COMPUTER-AIDED DESIGN: MICROSOFT CORPORATION $10,000 AWARD

Skin & bones, but plenty tough

Cannondale combines CAD and composites for a breakthrough bike

David J. Bak, International Editor

How will we manufacture bicycles in the next century? Better bikes--stronger, yet lighter. For two years Tod Patterson pondered this question, scrutinizing a variety of processes, materials, and concepts--some crazy, some not.

"Simply put, we're approaching the limits of aluminum tubular manufacturing," explains Cannondale's Special Projects Coordinator. "We can't take any more money or time out of the process."

Nor weight. Bicycle frame designers are like people with anorexia nervosa: They're obsessed with weight loss. Balancing pounds against strength, stiffness, and the other parameters that define a bicycle's ride characteristics, however, isn't easy. That's why the bike industry--which claims more than its share of clever, non-conformist engineers--rarely introduces a clever, non-conformist frame.

Were it not for a visit to the nearby Sikorsky Helicopter plant, Patterson too, might still be perplexing over the frame-maker's holy grail.

20% lighter. Inspiration came from above in the form of a Blackhawk helicopter. Surface skins built off a skeleton, Patterson was told, define the copter's basic shape. "From that moment on, I knew exactly how I wanted to build the bike," he recalls.

Patterson's revelation, like all good revelations, was simple yet profound: The skeleton absorbs in-plane loads like the rider's weight, front braking forces, and frontal impact forces. Skins handle side-to-side as well as torsional loads.

Cannondale's Super V Raven is the result. Introduced to the public last summer, the Raven shaves close to a pound off the frame weight of its predecessor--the company's Super V aluminum tubular frame. At only four pounds, the new frame features a vertically arrayed spine of investment-cast aluminum vertebrae. Left and right carbon- fiber skins sandwich the spine; epoxy resin bonds their edges into corresponding channels along the vertebras' lateral faces.

"Vertebrae subjected to higher loads have a thicker cross-section," Patterson explains. "Those in less highly stressed areas are thinner and lighter."

The skeleton also provides all the hard points for mounting components such as the fork, swing arm, and pedals. Skins are therefore left intact. This means there are no holes drilled through the fibers to disrupt load paths.

Because of their open section, the carbon-fiber skins can be manufactured by a number of molding methods including matched metal, compression, or transfer. Such processes allow high production rates with better control and accuracy when compared to monocoque construction techniques that call for bladders or cores to achieve hollow sections.

The latter may yield beautiful shapes but are often structurally weak. As the bladder expands, the composite shifts unpredictably. Proper coverage, therefore, requires excess carbon fiber, which can lead to overlapping and subsequent delamination.

CAD is critical. With the ability to diecast the skeleton and stamp the skins, Cannondale eliminates many of the cost variables associated with tubular frame design: welding, heat treating, and straightening, to name a few.

Without the benefit of 3D CAD, however, the skeleton/skin concept would not be practical. Computer-aided design not only creates the necessary composite shapes based on sizing data, weight, and strength requirements, it also generates the numbers needed for stereo-lithography (SLA) prototyping and mold making.

Patterson and his colleagues at Cannondale currently run Pro/Engineer from Parametric Technology Corp. (Waltham, MA) on Windows NT 4.0. They use Gateway G6-200 PCs and Pentium Pro processors.

Why Pro/Engineer? "When we first installed the software seven or eight years ago," Patterson recalls, "Pro/Engineer offered better parametric capabilities than competing packages."

Patterson explains that the bicycle industry offers many different frame sizes per model. "We wanted a software package that would allow us to design one style bike, and from that style modify the numbers to generate all the different sizes and update the respective CNC files."

Originally, Cannondale ran Pro/Engineer on UNIX platforms. They switched to Windows four years ago to network the company's different divisions and their various word processing, spreadsheet, and other software programs. "Because you could not easily network the UNIX machines with the rest of the company," Patterson says, "people like myself would end up with two computers."

  Connecting the dots. Today, one computer sits on Patterson's desk and he can share the design work he performs on it simultaneously around the company. With reference to the Super V Raven frame, here is what the other Cannondale employees would see:

Sizing data starts the process. Suspension points, head tube height/diameter, seat and pedal crank positions--these criteria, and their placement relative to each other, define the basic frame geometry.

The process is iterative. Using the solids modeling and surface capabilities of Pro/Engineer, Cannondale engineers can generate a three-dimensional frame shape for input into PTC's Pro/Mechanica finite element analysis program. Based on FEA results, the engineers can then go back and alter the original frame shape to optimize stiffness or other properties.

Left and right skin sides define the skeleton's size and shape. CAD information, downloaded to a stereolithography system, permits rapid prototyping. "From SLA," Patterson explains, "we investment cast aluminum parts in which we bond the skins to create a prototype frame. We actually ride test the prototype and ultimately take it through destructive testing," he reports.

Strain gages, placed on various points of the skins, help engineers correlate real-world tests with the strains predicted by FEA. Numbers for the Raven, Patterson claims, were within 5% of each other.

For Cannondale, the Raven's success ushers in a new era of bicycle frame manufacturing techniques. Patterson and his colleagues are presently looking into the use of thermoplastic vs. thermoset materials to speed cycle times and reduce production costs. They are also turning their sights on a matching skeleton/skin swing arm to cut weight even further. Obsessions, after all, are not easily satisfied.